Difference between Gases and Vapours

[danago]

What is the difference between a gas and a vapour? I remember one of my lecturers saying that there was a slight technical difference, but i don't think he ever did explain it because it was not important for what we were doing.

I did a quick google search and one of the results i found was that 'vapour' refers to a substance in the gaseous phase even though under normal conditions it does not exist as a gas i.e. the vapour pressure of a liquid/solid at standard conditions, whereas a 'gas' refers to a gaseous substance that does naturally occur as a gas under standard conditions, such as oxygen or nitrogen.

Is this distinction correct? The source wasn't the most reliable of sources, so it would be nice to get some validation

Thanks,
Dan.

 

[HallsofIvy]

A gas is one of the states of matter: solid, liquid, and gas. Anything can be anyone those depending on the temperature but things like oxygen, carbon dioxide, etc. are gases at "standard" temperature and pressure. "Vapor", however, is liquid droplets suspended in air.

 

[drizzle]

sorry off topic, but isn't the plasma considered as a fourth state of matter?

 

[MATLABdude]

sorry off topic, but isn't the plasma considered as a fourth state of matter?

And a Bose-Einstein condensate is considered a fifth, thus allowing you to span the entire temperature spectrum.

 

[drizzle]

And a Bose-Einstein condensate is considered a fifth, thus allowing you to span the entire temperature spectrum.

good, where can I read a good description of Bose-Einstein condensate? I would like to know, thanks in advance

 

[Borek]

"Vapor", however, is liquid droplets suspended in air.

Yes and no. When we speak about vapor pressure (or saturated vapor pressure) we think about gas.

 

[MATLABdude]

good, where can I read a good description of Bose-Einstein condensate? I would like to know, thanks in advance

I'd start with Wikipedia for a reasonable, high-level intro:
http://en.wikipedia.org/wiki/Bose-Einstein_condensate

 

[danago]

Oh, so when we talk about a vapour, it technically isn't in the gaseous phase at all then? Its technically still a liquid, just in a form that appears as a gas?

 

[D H]

"Vapor", however, is liquid droplets suspended in air.

That's not right. The liquid droplets suspended in the air are condensed water vapor. Condensed water vapor = clouds, uncondensed water vapor = humid air. Big difference.

A vapor is a substance that is in the gaseous phase but whose temperature is below the substance's critical point.

 

[danago]

That's not right. The liquid droplets suspended in the air are condensed water vapor. Condensed water vapor = clouds, uncondensed water vapor = humid air. Big difference.

A vapor is a substance that is in the gaseous phase but whose temperature is below the substance's critical point.

So in the case of water, any steam that exists below ~374 degrees celcius would technically be considered a vapour?

 

[Mapes]

"Vapor", however, is liquid droplets suspended in air.

Whoa! As D H, says, vapor is a gaseous phase.

(So why would we have two words for the same thing? People often use "vapor" to describe a gas in equilibrium with the corresponding liquid or solid phase. But others call this "saturated vapor" to be precise, and say "vapor" to imply only that the liquid or solid phase is present.)

 

[Ygggdrasil]

And a Bose-Einstein condensate is considered a fifth, thus allowing you to span the entire temperature spectrum.

You could also look at varying the pressure and include things like supercritical fluids as a state of mater as well.

 

[ank_gl]

When we asked this question in the class, prof gave us a reasonable answer. Gas is something which obeys the ideal gas laws, whereas vapor actually dont, while both are in gaseous phase.

 

[Borek]

When we asked this question in the class, prof gave us a reasonable answer. Gas is something which obeys the ideal gas laws, whereas vapor actually dont, while both are in gaseous phase.

As long as there are no traces of condensed liquid, vapor obeys ideal gas law as good as any gas does - classic example will be calculation of amount of water vapor in the equilibrium with liquid. Once the droplets appear, ideal gas law is no longer applicable to the whole system (although it still describes gaseous phase).

 

[Mapes]

When we asked this question in the class, prof gave us a reasonable answer. Gas is something which obeys the ideal gas laws, whereas vapor actually dont, while both are in gaseous phase.

Unfortunately, this is something either your professor made up, or it's a convention in extremely limited use. Vapor in many cases is modeled perfectly accurately with the ideal gas law. An example is the two huge fields of physical vapor deposition and chemical vapor deposition in microfabrication, where the vapor is typically at relatively low pressure and behaves ideally.

 

[ank_gl]

Unfortunately, this is something either your professor made up, or it's a convention in extremely limited use. Vapor in many cases is modeled perfectly accurately with the ideal gas law. An example is the two huge fields of physical vapor deposition and chemical vapor deposition in microfabrication, where the vapor is typically at relatively low pressure and behaves ideally.

engineering approximations

 

[chemisttree]

It depends upon how you define 'vapor'.

eg.

1. Barely visible or cloudy diffused matter, such as mist, fumes, or smoke, suspended in the air.
2.
a. The state of a substance that exists below its critical temperature and that may be liquefied by application of sufficient pressure.
b. The gaseous state of a substance that is liquid or solid under ordinary conditions.

http://www.thefreedictionary.com/vapor

It seems that 'vapor' is an imprecise term.

 

[Topher925]

If you had 1L of liquid water at 25C and 0 Pa in 2L tank with nothing else, would water vapor be present?

 

[Mapes]

It seems that 'vapor' is an imprecise term.

A few more vapor definitions: http://books.google.com/books?id=GC...vapor&lr=&as_brr=1&ei=33g6Sq_QKKGeygSGir26BQ". The scientific definition would seem to match the second part of the popular definition (a situation analogous to the scientific and popular definitions of the word theory).

If you had 1L of liquid water at STP in 2L tank with nothing else, would water vapor be present?

Yes. The second law tells us that there's a tremendous driving force for water molecules to evaporate, as this increases total entropy (up to the saturation pressure).

 

[Topher925]

Yes. The second law tells us that there's a tremendous driving force for water molecules to evaporate, as this increases total entropy (up to the saturation pressure).

I'm not sure I agree with this. Just because there is a mechanism for an increase in entropy doesn't necessarily mean it will cause a spontaneous reaction. Wouldn't the water molecules first require some sort of catalyst, such as dust or some sort of particle, to actually separate from the liquid and become a vapor. For example, the same way you can super heat water. If there is nothing to help trigger the phase change of water going from liquid to gas then the water will not boil. Wont this same phenomenon be observed with a saturated liquid at STP?

 

[Borek]

Superheated water doesn't boil, but it evaporates fast at the surface.

 

[Mapes]

I'm not sure I agree with this. Just because there is a mechanism for an increase in entropy doesn't necessarily mean it will cause a spontaneous reaction. Wouldn't the water molecules first require some sort of catalyst, such as dust or some sort of particle, to actually separate from the liquid and become a vapor. For example, the same way you can super heat water. If there is nothing to help trigger the phase change of water going from liquid to gas then the water will not boil. Wont this same phenomenon be observed with a saturated liquid at STP?

Well, the "trigger" needed to freeze, condense, boil, and precipitate is actually a requirement to nucleate a cluster of many atoms containing the new phase, as there's an energy penalty to creating additional surface area at a interphase interface. A larger cluster ameliorates this penalty because its surface-to-volume ratio is lower. But this constraint doesn't apply to surface evaporation because the interface area doesn't change. As far as I know, there's little or no activation energy required for a single atom or molecule to evaporate at a surface.

But if you find contradictory info in the literature, please correct me with a citation!

 

[Topher925]

Well, the "trigger" needed to freeze, condense, boil, and precipitate is actually a requirement to nucleate a cluster of many atoms containing the new phase, as there's an energy penalty to creating additional surface area at a interphase interface. A larger cluster ameliorates this penalty because its surface-to-volume ratio is lower. But this constraint doesn't apply to surface evaporation because the interface area doesn't change. As far as I know, there's little or no activation energy required for a single atom or molecule to evaporate at a surface.

That makes sense I think. But given the Clausius-Clapeyron relation, doesn't this require the vapor and the liquid to exist at two different temperatures since the evaporation would be an isoenthalpic process?

EDIT: I made a mistake in my previous post. I meant 25C and 0 Pa since the open volume would initially only contain a liquid. I now realize that this would in fact cause the water to boil. I'm not sure at what temp water exists as a liquid at 0 pressure, I'll have to look it up.

 

[Mapes]

That makes sense I think. But given the Clausius-Clapeyron relation, doesn't this require the vapor and the liquid to exist at two different temperatures since the evaporation would be an isoenthalpic process?

I'm not quite sure what you mean here. The Clausius-Clapeyron relation compares two equilibrium scenarios, each with a vapor and a liquid at the same pressure, the same temperature, and the same Gibbs free energy.

I'm not sure at what temp water exists as a liquid at 0 pressure, I'll have to look it up.

Can it exist? I'd think that at zero pressure vapor would be more stable at any temperature. I'd be interested to hear about what you find.